Research and Implementation of Screen Soft Proofing Technology
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In recent years, with the continuous development and improvement of color management and display technologies, soft proofing technology has gradually matured and has been promoted and applied in the printing industry. Soft proofing can greatly shorten the printing production cycle, reduce duplication of labor, cut costs, and improve efficiency. Soft proofing has fundamentally changed the traditional proofing mode, enabling printing companies and their printing suppliers to deliver proofs, ie remote soft proofing, over the network at extremely fast speeds. In the past proofing process, 98.6% of the time was spent sending samples, and only 1.4% of the time was spent on the sample body. With remote soft proofing, Chinese printing companies can overcome the disadvantages of geographical separation from foreign customers and create a faster circulation cycle. Both customers can realize real-time interaction, and the modification of proofs becomes simple and fast. Therefore, soft proofing not only realizes the so-called “what you see is what you get” concept, it also brings a more effective way of communication, a more effective process and production management philosophy.
First, the working principle of screen soft proofing technology
Soft proofing of the screen is a proofing method that simulates the display of the printed output on the display. Through color correction and color management of the display, the display effect is consistent between the displays, and the color displayed by the display is consistent with the color reproduction effect of the printed product, which is intuitive, convenient, flexible, cost-effective, and improves production efficiency. The advantages.
The basic principle of screen soft proofing is shown in Figure 1.
Figure 1 How the screen soft proof works
The soft proofing process undergoes two color space conversions: one is the conversion of the source file color space to the target output device (printer) color space; the other is the conversion of the target output device (printer) color space to the display color space, such as Figure 2 shows.
Figure 2 Color space conversion of soft proofing
The entire soft proofing process is divided into three phases: First, the monitor and the press are calibrated. When the monitor is calibrated, a series of software and hardware for calibrating the monitor are required. After the software is installed and the hardware is connected, the monitor calibration software is turned on and the screen is turned on. The prompt information is used for the calibration operation; after the characteristic files of the display and the printing machine are generated, the printing machine characteristic file is edited so that the display effect of the standard document on the display is as consistent as possible with the printing effect; finally, the printing machine is adjusted so that The print reproduction effect of the image is consistent with the display effect on the display to which the display and the printer profile are applied.
The premise of the soft proofing of the screen is that a high-quality display must be provided, and the cathode ray tube display and the liquid crystal display used are required to be no less than 17 inches in size and a resolution of not less than 1024 x 768 dpi. In recent years, with the continuous development of display technology, and the advantages of low energy consumption, low radiation, high color rendering stability, and declining price of liquid crystal displays, liquid crystal displays have gradually become mainstream displays for soft proofing. At the same time, the soft proofing of the screen is also based on color management. In short, the ideal screen soft proofing effect is based on high-quality display, with color management technology as the key, and precision measuring instruments and software as the guarantee.
Second, the realization of screen soft proofing technology
1. Display selection
If you want to get accurate screen soft proofing, you must use a professional monitor. The better the monitor, the better the color rendering. The gamut of the soft proof display must be larger than the simulated print gamut, which is a prerequisite for the validity and feasibility of soft proofing. Only in this way can the complete and accurate reproduction of the gamut be guaranteed. At present, professional monitors for soft proofing include Apple monitors, EIZO monitors, and the like.
The color gamut of the display: Most of the ordinary displays are RGB, and the general blue-green display is not particularly good. Today's high-end displays generally use the Adobe RGB color gamut, which has almost the same requirements as four-color printing.
The contrast ratio of the display: the contrast ratio of the high-end display is generally between 350:1 and 600:1.
The stability of the display: the stability of the display has a great influence on the display. The high-end display should have a self-stabilizing display function, and can quickly enter a stable working state at the startup.
2. Correction of the display
The accurate correction of the display is the basis for realizing the soft proofing of the screen. The calibration of the display is the process of adjusting the display to the standard state. The display characteristic of the corrected display conforms to the ideal parameter value set in its own device description file, so that the display card is based on the image data. The color data shows the color accurately on the display.
The calibration of the display must first standardize the placement environment and lighting conditions of the display to ensure the accuracy of color reproduction. Secondly, it is necessary to ensure that the calibration of the display is performed under the stable working state of the display, that is, sufficient preheating is performed before the correction; Third, when performing display calibration, the correct setting of the parameters is the most important. It is necessary to set the parameters such as gamma value, white field color temperature, brightness/contrast, etc. according to the type of the display, the current state, etc.; Display hardware and color rendering performance will fluctuate over time, so the display should be calibrated periodically.
The following is a detailed description of the calibration process of the display with a combination of Eye-one Pro spectrophotometer, EIZO display hardware and ProfileMaker 5.08 software. Open ProfileMaker/Select Monitor item, where you can calibrate the display and create the ICC for your display. In the Reference, select the type of display you want to correct: CRT (Cathode Ray Tube); LCD (Liquid Crystal Display). Select Eye-one Pro in the Sample and follow the on-screen instructions to take measurements step by step with Eye-one Pro. Firstly, the color temperature, gamma value and brightness of the display are correctly set, and then the brightness, contrast and RGB three-color values of the display are adjusted separately to make the display reach the optimal state, thus completing the calibration of the display. After the calibration is completed, the ICC measurement of the screen is performed by Eye-one Pro, and the ICC profile of the display is finally obtained through calculation in ProfileMaker.
3. Color management is the core of screen soft proofing
How to make the result of soft proofing better reflect the effect of printing output has been paid more and more attention by people. To make the color faithful reproduction of the screen soft proofing, it is necessary to apply color management technology. The soft proofing of the screen is based on the color management. On the top, it realizes the characterization of devices such as monitors, printing presses, scanners, etc., and realizes the mutual simulation between the color gamut of the display and the color of the color field of the printing machine, ensuring the consistency of colors on different devices. The key to achieving high quality soft proofing.
Color management is a technique for correct interpretation and processing of color information. It is a method of converting the color data of an image from one color space to another under the premise of ensuring the minimum color distortion to ensure the color of the same image. The appearance of the input, display, and output processes is as close as possible and consistent. Color management is generally divided into three processes: device calibration, characterization, and color conversion. The color management system uses the CIE Lab chromaticity space as a reference color space to record the color of the input device or the output device with the feature file, and uses the application software and third-party color management software as the user's color control tool. The basic structure of an ideal open color management system is shown in Figure 3.
Figure 3 Schematic diagram of the color management system
Implementation of color management: 1 select a device-independent space as the converted reference color space; 2 calibrate the device; 3 device color description (description); 4 color space conversion between devices (Conversion) ); 5 Color Gamut Mapping between devices.
Third, the screen soft proofing in Photoshop
Imageshop software Photoshop is widely used in digital prepress operations and plays an important role in digital image restoration and color correction. The software not only has ICC-based color management capabilities, but also provides soft proofing capabilities for analog hard copy output.
Soft proofing of images in Photoshop, first of all, color conversion, when you want to preview the output file, you need to complete two color conversions – from the color space of the image to the color space of the output device to be previewed; The color space is converted to the color space of the display.
Screen soft proofing in Photoshop is mainly done through the Poof Setup dialog in the View drop-down menu. The specific soft proofing process is as follows:
Under the premise of correcting the display, open the Color Check24 standard color target image in LAB mode in Photoshop. This standard plate contains 24 color blocks, covering most of the colors in the color gamut, and many of the color blocks represent the memory. Color and neutral colors, such as skin color, plant and blue sky colors, can accurately evaluate the soft proof display.
Figure 4 Color Check24 standard color target map
Select Proof Setup from the View drop-down menu and select the Custom command from its submenu. A Proof Setup dialog will pop up. This dialog has powerful softness. Proofing function.
Figure 5 Proof setting dialog
In the "Profile" menu of the dialog box, specify the profile of the output device to be simulated on the screen. In its drop-down menu, you can select the description file of almost all devices. If you want to achieve more accurate results, you can load your own. The properties file of the output device.
The “intention” menu refers to the gamut matching method that converts from the color space of the printing press to the color space of the screen. Four matching methods can be selected: absolute chromaticity colorimetric, relative chromaticity colorimetric, perceptual colorimetric and saturation ratio. color. The color gradation transformation for absolute chromaticity purposes includes color adaptation, no white point adaptation, and the gamut mapping adopts the gamut clipping method. Because it does not perform white point adaptation, it is suitable for simulating another device on one device. effect. The absolute chroma is the best choice when the screen is soft proofed, so the parameters in the dialog should select the absolute chroma rendering intent, and the simulated paper white item should be checked. The effect of simulating the paper white is to simulate the image when the monitor previews the image. The color of the paper at the time of printing specified by the device property file is output, and the analog ink black option is automatically selected when the simulated paper is selected. This displays the actual dynamic range defined by the device output file, enabling accurate soft proofing.
After the parameters of the dialog box are set, the image is soft proofed. The color difference between the color block measurement value and the standard in the Color Check24 standard color target image is shown in Table 1.
Table 1 Color difference statistics between measured values and standard values
From the above experimental data, it is concluded that in Softshop, the average color difference of the color patches is 4.82, and the color patches with the color difference of less than 10 account for 87.24%. International standards stipulate that for fine prints, the color difference is not noticeable within 6 and is acceptable for ordinary observers. Therefore, accurate proofing of the screen can be achieved by using the proof dialog box in Photoshop.
Fourth, the conclusion
Screen soft proofing is a brand new proofing method. Its many advantages make it more and more people's attention, especially with the advantage of the network, it can realize the function of remote soft proofing, which can provide more valuable for printing. The information facilitates communication with customers, enables customers to understand production status in a timely manner, view production processes at any time, correct errors, avoid waste and repetitive work, reduce production costs, and improve work efficiency. At present, more and more developers and technicians in China have researched and developed screen soft proofing technology. Soft proofing has become more and more accurate in color matching, and the cost is cheaper. Soft proofing has replaced traditional proofing. Become an inevitable trend.